1. Field of the Invention
The present invention relates to a technology to join dissimilar materials, or materials with different melting points, for example, such as a steel material and an aluminum alloy board. Specifically, the present invention relates to a method of joining dissimilar materials which joins a high melting point material and a low melting point material in the form of a line by irradiating the surface of the high melting point material with a high-energy beam while moving the high-energy beam, and relates to a joining machine used in such joining and a joint structure of the dissimilar materials joined by such a joining method.
2. Description of the Related Art
Generally, in joining dissimilar materials, if the both materials to be joined are melted in a similar way to a case of welding similar materials, a brittle intermetallic compound is produced. Accordingly, sufficient joint strength cannot be obtained between the dissimilar materials. For example, in the case of joining aluminum alloy and steel, which are dissimilar metals, a hard and brittle intermetallic compound, such as Fe2Al5 and FeAl3, is produced. In order to ensure sufficient joint strength, such an intermetallic compound therefore needs to be controlled.
However, there is a dense and strong oxide film formed on the surface of the aluminum alloy. Removal of the oxide film requires a large amount of heat to be given to the joining, which results in growth of a thick intermetallic compound layer. The joined portion is therefore low in strength.
As described above, joining the dissimilar materials needs to be carried out with the growth of the intermetallic compound being precisely controlled. Accordingly, a method using a high-energy beam which allows precise temperature control, such as an electron beam or a laser beam, as an external heat source for heating has been attempted.
To control such production of the brittle intermetallic compound, the lap joint of the dissimilar materials using the high-energy beam is achieved by the following method. First, the high melting point material is irradiated with a high-energy beam defocused. The low melting point material is then melted by heat transfer from the high melting point material, so that the high and low melting point materials are joined to each other (see “PREPRINTS OF THE NATIONAL MEETING OF JAPAN WELDING SOCIETY”, Japan Welding Society, p. 152, Vol. 72, April 2003). In this method, welding conditions are controlled so that only one of the materials (low melting point material) is melted in the joint interface to achieve the joint using diffusion of the material. The growth of the intermetallic compound layer can be therefore suppressed, and the thickness thereof can be thin. Accordingly, the strength per unit area at the joint is thought to be able to be made higher than that in the case of joint by melting the both materials.